1. Field of the Invention
This invention relates to a novel optically active pyridylethanol derivative and a liquid crystal composition containing the same.
2. Description of the Related Art
As to liquid crystal display elements, their use applications have been rapidly broadened due to improvements in circuits, driving modes and cell preparation techniques and particularly due to improvement in characteristics of liquid crystal compositions filled in the elements. However, current liquid crystal display elements still have a number of drawbacks to be overcome, such as narrow angle of view, inferior contrast, low response rate, still yet small display capacity, deterioration in the display quality due to change in ambient temperature, etc. Among these, the deterioration in the display quality due to change in ambient temperature is said to originate in the temperature change of threshold voltage (Vth).
In recent years, as a means usually employed, there has been known a method of adding a slight quantity of an optically active substance to a liquid crystal composition to thereby suppress the reverse twist of the liquid crystal molecule and impart a right-handed or left-handed helical structure to the liquid crystal molecule to retain the display quality. Hence, in the case of TN (Twisted Nematic) mode display elements and recently proposed SBE (Super twisted Birefringence Effect) mode display elements, etc., the above-mentioned method can have an advantageous influence. However, if the twistability of the optically active substance to be added as a dopant is too low, it is necessary to add the substance in a relatively high concentration in order to obtain a required pitch; hence it is noted that this has an influence upon other substance parameters. Thus, an optically active substance having a high twistability, i.e. when added to liquid crystals, having a capability of exhibiting a shorter pitch in the same quantity added, has been long awaited. For example, in the case of CB-15 made by BDH Company or compounds disclosed in Japanese patent application laid-open Nos. Sho 62-81354/1987 and Sho 62-81355/1987, when these compounds are added in an amount only one percent by weight to nematic liquid crystal compositions, they have a capability of exhibiting a pitch of about 10 um; hence they can be practically usable, optically active substances in a certain sense.
However, generally known optically active substances including the above-mentioned compounds generally vary the pitch with temperature change and often have an undesirable influence. For example, in the case of SBE mode, the intrinsic pitch P of the liquid crystal composition varies with temperature change and thereby the ratio of the intrinsic pitch P of the liquid crystal composition to the cell thickness d of the display element (P/d) also varies. P/d is usually 2 or less, but if it exceeds 2, 270.degree. twist changes to 90.degree. twist. Further, from the aspect of improvement in the increase of display capacity, it is necessary to improve the steepness of change in the transmittance in the case where voltage is being impressed onto display elements. G. Bauer and W. Fehlenback reported a calculation result that 270.degree. twist notably improves the steepness (the 15th Freiburg liquid crystal symposium (1985), but even in this case, it is necessary to be free the composition of change in the intrinsic pitch dependent on temperature change.
As a method for solving this problem, it has been reported in recent years that when an optically active compound having a negative temperature characteristic, i.e. a substance the intrinsic pitch of which is reduced with temperature rise, is blended with an optically active compound having a positive temperature characteristic, i.e. a substance the intrinsic pitch of which is increased with temperature rise, each in an adequate quantity, then a composition free of change in the intrinsic pitch depending on temperature is obtained(Japanese patent application laid-open No. Sho 63-22893/1988).
However, since two kinds of optically active compounds, i.e. a substance having a positive temperature characteristic and that having a negative one, should be blended together, this requires complicated steps such as a search for adequate blending ratios.
Furthermore, any of currently reported substances having a negative temperature characteristic have a low twistability; hence in order to obtain a required pitch, it is necessary to add the substances in a considerably high concentration so that this appears to have various bad influences. Thus, in order to solve such various problems, there has been desired a substance having a short pitch and yet having a superior temperature characteristic, in short, free of change in the pitch with temperature change.